Multi-tiered-recess screws

ABSTRACT

A screw ( 10 ) and driver ( 30 ) system comprises a range of screws and a driver. Each screw has a head ( 12 ) having a driving recess ( 14 ) in its surface ( 13 ) for engagement by the driver. Each screw has a longitudinal axis ( 100 ). The recess ( 14 ) of larger screws in the range have a plurality of superimposed recess-tiers ( 16   a,b,c ) of decreasing size with increasing depth from said surface. Each recess-tier has substantially parallel driving surfaces. They are substantially parallel the longitudinal axis. The driver and recess are shaped so that, when the driver is engaged with the recess of any screw in the range, torque applied to the driver is transmitted to the screw through said driving surfaces.  
     At least one recess-tier ( 16   c ) of at least smaller screws in the range has a rib ( 20 ) which is parallel the longitudinal axis ( 100 ) and encroaches into the space of the recess-tier occupied by the driver when it is engaged with the recess. Engagement of the driver with the recess causes deformation of rib, and hence creation of an interference fit of the driver in the recess.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of co-pending application Ser. No. 10/703,115, filed Nov. 6, 2003, entitled “Multi-tiered-recess screws”.

BACKGROUND OF THE INVENTION

This invention relates to multi-tiered-recess screws, that is to say, screws comprising a driving recess for insertion of a driving tool, which recess comprises a plurality of superimposed recess-tiers of decreasing size. The recess-tiers may be concentric, in which event they are non-circular. Indeed, the invention is particularly concerned with the latter, because these have the additional feature that, whereas the driver has a fixed number of tiers, the screw may have some or all of the recess-tiers, depending on its size and torque driving requirements.

It is a particular feature of this kind of screw that there is a single driver that is suitable for driving a range of sizes of screw. Smaller screws simply have one or two small recesses, while the larger screws have larger recesses also.

GB-A-1150382 appears to be the first disclosure of a screw provided with a multi-tiered recess and a corresponding multi-tiered driver. GB-A-2285940 discloses essentially the same idea. Both these publications describe the advantages provided by the arrangements disclosed. The first is that the recesses are essentially parallel-sided and consequently eliminate cam-out problems that are associated with cross-head recesses. Secondly, they give the possibility of a single driving tool being suitable for driving a wide range of screw sizes.

The single driving tool typically has three (for example) tiers of driving surfaces which are employed to drive large screws having three recess-tiers of recess. However, the same tool can be employed with smaller screws having only two recess-tiers of recess, the largest recess-tier being omitted. Indeed, even smaller screws may have only one, the smallest recess-tier, in their recess and be driven by the smallest tier only of the tool.

GB-A-2329947 discloses a similar arrangement, and WO-A-0177538 discloses recess-tiers that have such a small extent in the recesses of screws and bolts that, at the torques at which the screws are intended to be operated, they cannot be turned unless at least two recess-tiers are both engaged by the tool. Otherwise, the screw is arranged to round out of engagement with the driving tool. This provides a security feature in that only the appropriate tool, having all the requisite driving tiers, will undo the screw.

WO03/025403 discloses a method of manufacture of such screws using cold forming punches. It is possible to make the recesses with some precision, so that the driving tool is a close fit in the recess. This has the very useful feature that recess-tiers can be shallow. Then, screw heads do not need to be large to accommodate the driving tool. Yet, adequate torque can still be applied because a large proportion of the area of each recess is used for torque transmission by virtue of the close tolerance fit. But, equally usefully, the tool fits the screw so closely that, once mated with the driving tool, the screw can be carried solely by the driver when it is offered up to a workpiece. Indeed, with self-tapping wood screws, the connection between driver and screw is so stable that some pressing and simultaneous rotation forces can be applied to the tool, without holding the screw. This can be done without significant risk that the connection will fold as may happen with, for example, Posi-Driv (registered trade mark) screws unless forces are maintained absolutely axial. With the three-tiered screws of the present applicants marketed under the registered trade mark Uni-Screw, the fit is so close that even carrying screws dangling vertically from the driver is possible if carried carefully.

Nevertheless, it would be desirable to improve this feature. This is particularly so with screws having only one or two recess-tiers of recess. It seems that it is partly the plurality of recess-tiers that, at least to some extent, explains why the screw appears to grip the driver so effectively. So, with smaller screws having just one recess-tier of recess the feature is not so evident. It is an object of the present invention to improve that feature.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a screw and driver system comprising a range of screws and a driver for said range, each screw comprising a head having a driving recess in its surface for engagement by said driver and a longitudinal axis, in which the recess of larger screws in the range have a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier having substantially parallel driving surfaces substantially parallel said longitudinal axis, and in which said driver and recess are shaped so that, when the driver is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces; wherein, an interference is provided between the driver and the recess causing deformation of the recess when the driver is inserted therein.

Preferably, said interference comprises at least one recess-tier of at least smaller screws in said range having a rib parallel said longitudinal axis and encroaching into the space of said recess-tier occupied by said driver when it is engaged with said recess, whereby engagement of the driver with the recess causes deformation of said rib and hence creation of an interference fit of said driver in said recess.

Preferably all the screws in the range have said rib. The rib may be in a recess-tier which is common to all screws in the range. Alternatively, the rib may be in the single recess-tier of single recess-tier screws, and a different recess-tier in screws having a plurality of recess-tiers.

The invention also provides a screw that fits in said range of screws in said system, which screw includes a recess-tier having said rib.

Preferably, said recess-tiers of said larger screws are non-circular polygons in section. The polygons may be concentric. They may be the same polygons. They may be angularly offset with respect to one another. The smaller screws in said range may have only one recess-tier in their recess.

Screws in accordance with the present invention are conveniently made by a cold forming process in which the recess is formed by a punch of corresponding shape to the recess being formed. In this event, preferably, said recess rib is formed by a groove formed in the flank of said punch.

Preferably, said rib is the full depth of only one recess-tier, where there are multiple recess-tiers.

One advantage of the present invention is that the rib can be large enough to ensure an interference fit with the driver but, when it is only in one recess-tier, it does not increase substantially the overall force required to engage the driver with the recess, particularly not on larger screws.

Preferably, it is the smallest recess-tier in each screw of the range that is provided with said rib. Indeed, it is on larger screws that the dimensions of the smallest recess-tier are most susceptible to fluctuation in dimensional tolerance.

Where the recess-tiers of the recess are polygonal in section having several flanks, the rib may be central in one flank. Alternatively, and this is preferred, the rib may be in a corner between two flanks. This has the advantage that a sharp corner of the driving tool bites into the rib more easily than the rib simply being flattened by the flank of the tool. Indeed, a smaller rib is preferably provided in each corner between the flanks of the recess-tier. This maintains the central location of the driving tool in the recess.

It is inherent in all screw and driver arrangements, whether of the type to which the present invention relates or otherwise, that the driver is constructed from a harder material than the screw. The reason for this is self-evident, in that a screw is driven once or twice, whereas a driver drives a multiplicity of screws. Therefore it is essential that screws wear preferentially compared with the driver.

Despite this, the constant engagement of part of the driver with ribs in successive screws will wear the driver in the region of its engagement with the ribs. It is therefore an option to vary the location of the rib between screws in the range so that different parts of the driver engage the rib, whereby wear of the driver is evened out.

In an alternative arrangement, said interference comprises at least one tier of the driver having a rib parallel said longitudinal axis, said rib encroaching into the space occupied by a wall of the corresponding recess-tier of a screw in said range and when the driver is engaged with said recess, whereby engagement of the driver with the recess causes deformation of said wall and hence creation of an interference fit of said driver in said recess.

Nevertheless, this alternative arrangement has the disadvantage that it is the same rib on the driver that interferes with every screw driven. Consequently the wear of the rib on the driver may ultimately cause the interference to disappear in time.

Accordingly, in a further alternative, the smallest tier of said driver comprises a distal end thereof and a proximal end, and said interference comprises a tapering of the cross-section of said smallest tier from said proximal to said distal end, the cross section of the tier intermediate said ends corresponding with the cross section of the smallest recess-tier of a screw in said range.

Therefore, when the driver is engaged with the recess of a screw, flanks of the walls of the smallest recess-tier are deformed creating an interference fit between them.

Likewise, the converse may be provided where the smallest recess-tier of the recess of each screw in said range has a bottom end and an open top end, said interference comprising a tapering of the cross-section of said smallest recess-tier from said open top end to said bottom end, the cross section of the recess-tier intermediate said ends corresponding with the cross section of the smallest tier of said driver.

Preferably, where said tiers/recess-tiers are polygonal in section and concentric, said tapering is around the entire periphery of the tier or recess-tier, as the case may be. In this way, there is no lateral displacement of the driver with respect to the screw when the driver is inserted, and also that wear on the driver is spread around the entire periphery of the affected tier. Furthermore, not only can the degree of insertion of the driver vary, but also the nature of the engagement is, or could be, more compressive and potentially elastic, rather than any plastic deformation of the screw. Both these factors will tend to reduce wear on the driver.

It is possible, although not preferred, that said tapering may be provided alternatively, or in addition, on other tiers of the driver, or recess-tiers of the screws. Between the smallest recess-tiers/tiers of the screw and driver, the tapering can be relatively great, ensuring that any tolerance in the dimensions of the recess-tier and driving-tier is taken up by the tapering and grip between the driver and screw is reliably effected every time. Nevertheless, the forces needed for insertion are not large, even if there happens to be a tight tolerance between any given driver and screw. Such would not be the case, however, with larger tiers where the force required would increase with the size of the interference between the driver and screw.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are further described hereinafter, by way of example only, with reference to the accompanying drawings, in which:

FIGS. 1A and B are a perspective view and side view, partly in section, of a screw in accordance with the present invention;

FIGS. 2A and B are similar views to FIG. 1, but of a different screw in accordance with the present invention;

FIGS. 3A, B and C are two side views and an end view in the direction of arrow C in FIG. 3B, respectively, of two punches to produce screws in accordance with the present invention;

FIG. 4 is a side view of a driver in accordance with the present invention;

FIGS. 5A and B are similar views to FIGS. 1 a and b of a different embodiment of the present invention;

FIGS. 6 a and b are similar views to FIGS. 2 a and b of another different embodiment of the present invention;

FIG. 7 is a side view of a driver in accordance with another embodiment the present invention; and

FIGS. 8A and B are side and section views of a driver and screw respectively of further different, but related, embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIGS. 1A and B, a screw 10 has a recess 14 in its head 12 which opens from the top surface 13 of the head 12. The recess 14 comprises three super-imposed hexagonal recesses 16 a,b,c, each of reducing dimension. The bottom recess 16 c includes a rib 20.

Turning to FIG. 4, a driver 30 comprises a shaft 32 and a driving tip 34 comprising three tiers 36 a,b,c. The cross sections of the driving tiers 36 a,b,c correspond with the cross sections of the recesses 16 a,b,c of the screw 10. However, the rib 20 intrudes into the space occupied by the tier 36 c of the driver 30 when the driver is engaged with the screw 10. Accordingly, when the driver 30 is inserted into the recess 14, the rib 20 is deformed by the tier 36 c so that an interference fit is created in which the recess-tier 16 c grips the tier 36 c of the driver. Then, the screw 10 is firmly seated on the end of the driver 30 so that the screw can be manipulated to any position without the risk of it falling off the driver 30.

In FIG. 2A, a larger screw 10′ is illustrated. It has a recess 14′ which comprises just two recess-tiers 16 a′,b′. Here, recess 16 b′ is deeper than the corresponding recess of the FIG. 1 arrangement and is at least equal to the combined depths of recesses 16 b,c of the FIG. 1 screw. The advantage of this arrangement is that the recess 16 a′ is also made deeper than that the corresponding recess-tier 16 a of the FIG. 1 screw. Therefore, more of the tier 36 a of the driver 30 is employed to drive the screw. Being a larger screw, greater torque is normally required and this is provided by the extra depth of the largest recess-tier 16 a′. Since the recess-tier 16 c is missing in this screw, a rib 20′ is provided in the recess 16 b′. Whereas such a long rib might appear to potentially create an excessive quantity of material to be deformed, it should be borne in mind that only its top half would be engaged by the tier 36 b of the driver 30, the bottom half of the recess-tier 16 b′ being occupied by the smallest tier 36 c of the driver 30. The reason why the bottom recess-tier is not provided here is because the torque contribution which it would make to the recess is negligible compared with the other two recesses. Instead, it is omitted so that it does not have the opportunity to fill and prevent full insertion of the driver 30.

Turning to FIG. 3, in FIG. 3A is shown a punch 40 having a recess forming head 42 comprising a single hexagonal tier 44 c. In the side of one face 46 of the tier 44 c there is provided a groove 48 whose function is to form the rib 20 in the recess of a single recess-tier screw (not shown) being worked by the punch 40. Likewise, in FIG. 3B, a punch 40′ has three tiers 44 a,b,c, and is for forming the recess 14 of the screw shown in FIGS. 1A and B. This likewise has a groove 48 for forming the rib 20. No punch is shown to form the screw of FIGS. 4A and B, or indeed the screws described below with reference to FIG. 8B. Nevertheless, the form of such punch is self-evident.

The diameter of the groove 48 is preferably about 3.0×10⁻³ inches in diameter (about 0.08 millimeters) and it has a depth of about half this amount.

An advantage of the present invention is that the rib 20 is only active over a small proportion of the length of engagement of a driver with a recess. At least, this is the case in connection with multi-tier recesses where the interference fit feature is not required to such an extent. With the smaller recesses, however, where it is more necessary in order to retain the screw on the driver, it occupies a greater proportion of the depth of the total recess. Thus, in smaller screws it takes on a greater role in holding the screw in place on the driver than is required in larger screws where the multiplicity of recess-tiers assists in this function.

While only one rib 20 is illustrated, it is of course feasible to provide a rib in each face or, in the case of hexagonal section recess-tiers, in every other face. With multiple ribs, the size of each rib would, of course, be less than the size of a single rib. The advantage of multiple ribs evenly distributed around the longitudinal axis (eg 100, FIG. 1B) of the screw is that this facilitates insertion of the driver in the recess 14 and helps to keep the driver central with respect to the recess.

In FIGS. 5A and B, instead of the rib being placed centrally in a flank 50 of the recess-tier recess 16 c″, here, the screw 10″ has a small rib 20″ in each corner between adjacent flanks 50. The driving tool 34 (see FIG. 4) has sharp corners 38 between its flanks, and these cut into the ribs 20″, making insertion of the tool easier. Nevertheless, there is still created the interference fit between the tool and screw.

FIGS. 6A and B show a modification where the single rib 20′″ is placed in just one corner in the recess-tier 16 b″. This has the same effect as the FIG. 5 arrangement, although may have the disadvantage, as does the FIGS. 1 and 2 arrangements described above, of tending to decentralise the tool when it is being inserted in the screw 10′″.

The ribs 20″ of the FIG. 5 arrangement are each smaller than the rib 20′″ of the FIG. 6 arrangement. Because the rib 20′″ is in a corner, where it will more easily be cut and spread by insertion of the tool, it can be thicker than the rib 20. Here it may be between 4 and 10×10⁻³ inches (between 0.1 and 0.25 mm) in depth from its surface to the corner (dimension d in the inset to FIG. 6 a). The ribs 20″ however, will each have a depth of between about 1 and 1.5×10⁻³ inches (0.025 and 0.05 mm).

Another advantage of the FIGS. 5 and 6 arrangements is that it is much easier to form the punches 40, 40′, because here the punch simply requires a corner of its tier 44 c (or all its corners on that tier in the case of the FIG. 5 arrangement) to be chamfered to the requisite degree. This avoids the problem of scoring the groove 48 in a flank of the tool.

In FIG. 7, an alternative arrangement is proposed, being the mirror image, essentially, of the proposal described above in relation to FIG. 1A. Here, instead of forming the rib 20 in the recess, it is (or they are) formed on the driver 30′ as ribs 120. This has precisely the same effect as the FIG. 1A,B embodiment, but it has the disadvantage that it is the same ribs 120 that are engaged with every screw that is driven. Consequently there is a tendency for the ribs 120 to wear and eventually to cease to be effective. This problem with this embodiment is, however, to a certain extent illusory, because the same parts of the driver always engage the rib 20, so, instead of wearing off the rib 120 on the driver 30′, the rib 20 of the screw simply wears a groove in the driver 30. Of course, this problem is minimised with the FIGS. 1 to 6 arrangements provided there is alternative faces/edges of the driver engaging the ribs. In this respect, a single rib 20 (FIG. 1A) divides the wear between six faces of the driver (assuming hexagonal recess-tiers 16). Likewise, a rib in every other face or in every other corner, divides the wear by two. In this respect, another option is to mix the locations of the ribs in the recess so that wear is spread around the driver.

Be that as it may, the driver is almost invariably made of harder material than the screws, particularly in the case of wood screws that are relatively soft, whereby wear preferentially occurs on the screws, rather than the drivers.

Finally, turning to FIGS. 8A and B, two similar but different arrangements are shown. In FIG. 8A, the bottom tier 36 c′ of a driver 30″ has a distal end 136 and a proximal end 138, the latter connected to the rest of the driver 30″. The cross-section (hexagonal, in this case) of the tier 36 c′ tapers from the proximal end to the distal end. In the drawing, it tapers in a curving manner, but it might equally, indeed preferably, taper in a straight manner. The dimensions of the cross-section of the tier 36′c, at some point intermediate the distal and proximal ends 136,138 of the tier 36 c′, are equal to the corresponding dimensions of the recess-tier 16 c (of a screw not having any rib, or indeed any tapering of its recess). Ideally, they are the same at or near the distal end, although this will vary depending on the tolerances of the recess.

Thus, as the driver is engaged in the recess of a screw, the tapering of the bottom tier 36 c′ of the driver progressively bites into the corresponding recess-tier of the screw. To begin with, it is just a gentle nipping of the flanks of the tier 36 c′. However, when the driver is fully engaged, there is a more substantial deformation of the edge of the recess-tier. This is still not substantial in the sense that significant force needs to be applied to fully engage the driver: this is not the purpose. Rather, it is to ensure secure gripping of the driver by the screw so that it can reliably be carried by the driver and does not come loose except by deliberate action. Nevertheless, an additional advantage of this feature is that tight engagement between the bottom tier 36 c′ of the driver and the corresponding recess-tier of the screw is ensured, despite any flexibility of the tolerances of the screw, and consequently the risk of the driver turning in the recess of the screw is reduced, particularly in connection with small, single recess-tier screws. It is true, of course, that there is a certain tendency by this arrangement to return to the camming-out problems which it is one feature of these multi-tier screws that is normally eliminated. However, the problem only occurs with smaller screws and the smallest recess where the axial force required to overcome any camming-out tendency is fairly minimal.

It is to be noted that this arrangement does not work, of course, with screws 10′ as shown in FIGS. 2A,B. However, there is no reason why these screws should not have the rib 20′ as described above, since there will be no duplication of the interference provided.

FIG. 8B illustrates the mirror arrangement to that described above in relation to FIG. 8A. Here, the tapering is provided in the recess-tier 16 c′″ of recess 14″″ of screw 10″″. The tapering is between a bottom end 116 of the recess-tier 16 c′″ and an open top end 118 thereof. With this screw, a normal driver 30 (FIG. 4) is employed and the cross-section of the driver tier 36 c is arranged to be only slightly less than the dimension of the open-end 118 of the recess-tier 16′″. In this way, as the driver is inserted, its tip 36 c becomes progressively squeezed by the tapering recess-tier 16 c′″.

Thus, in a preferred arrangement: the diameter (that is, flat to flat dimension) of the largest tier 36 a of the driver 30″ is 5.9 mm, with a minimum depth of 3.1 mm; the diameter of the middle tier 36 b is 3.9 mm, with a depth of 1.1 mm; the diameter (not D, which is edge to edge) of the proximal end 138 of the smallest tier 36 c′ is 2.6 mm; the diameter (likewise, not d) of the distal end 136 of the smallest tier 36 c′ is 2.4 mm; and, the depth of tier 36 c′ is 1.1 mm. With this arrangement, the diameter of the smallest recess-tier of the corresponding screw is 2.5 mm throughout its depth.

If, on the other hand, it is the screws (ie screw 10″″) that are provided with the tapering recess-tier: the diameter (that is, flat to flat dimension) of the largest recess-tier 16 a is 5.9 mm, with a depth of 1.5 mm; the diameter of the middle recess-tier 16 b is 3.9 mm, with a depth of 1.1 mm; the diameter (not D′, which is edge to edge) of the open end 118 of the smallest recess-tier 16 c′″ is 2.6 mm; the diameter (likewise, not d′) of the bottom end 116 of the smallest recess-tier 16 c′ is 2.4 mm; and, the depth of recess-tier 16 c′″ is 1.1 mm. With this arrangement, the diameter of the smallest tier of the driver is 2.5 mm throughout its depth.

Although the tapering is shown and described as being applied to all driving faces or flanks of the tier 36 c′ or recess-tier 16 c′″, it is not essential that each be flared in this way. In fact only one could be flared, although this would lead to some imbalance. Thus only every other face may be flared, as preferred. 

1. A screw and driver system comprising a range of screws and a driver for said range, each screw comprising a head having a driving recess in its surface for engagement by said driver and a longitudinal axis, in which the recess of larger screws in the range have a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier having substantially parallel driving surfaces substantially parallel said longitudinal axis, and in which said driver and recess are shaped so that, when the driver is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces; wherein, an interference is provided between the driver and at least one recess-tier causing deformation of the recess when the driver is inserted therein.
 2. A system according to claim 1, wherein said interference comprises at least one recess-tier of the screws in said range having a rib parallel said longitudinal axis and encroaching into the space of said recess-tier occupied by said driver when it is engaged with said recess, whereby engagement of the driver with the recess causes deformation of said rib and hence creation of an interference fit of said driver in said recess.
 3. A system according to claim 1, wherein each said recess-tier of said screws is a non-circular polygon in section.
 4. A system according to claim 3, wherein, in said larger screws, said polygons are concentric.
 5. A system according to claim 4, wherein said recess-tiers are the same polygons.
 6. A system according to claim 5, wherein said recess-tiers are angularly offset with respect to one another.
 7. A system according to claim 2, wherein said rib is in a recess-tier which is common to all screws in the range.
 8. A system according to claim 2, wherein said rib is in the single recess-tier of single recess-tier screws, and a different recess-tier in screws having a plurality of recess-tiers.
 9. A system according to claim 2, wherein said rib is the full depth of only one recess-tier, where there are multiple recess-tiers, or of the recess-tier, where there is only one recess-tier.
 10. A system according to claim 2, wherein said polygonal recess-tiers have flanks, said rib being central in one flank.
 11. A system according to claim 2, wherein said polygonal recess-tiers have flanks, the rib being in a corner between two flanks.
 12. A system according to claim 11, wherein a rib is provided in each corner between the flanks of the recess-tier.
 13. A system according to claim 2, except claim 8, wherein it is the smallest recess-tier in each screw of the range that is provided with said rib.
 14. A system according to claim 1, wherein the smaller screws in said range have only one recess-tier.
 15. A system according to claim 1, wherein said interference comprises at least one tier of the driver having a rib parallel said longitudinal axis, said rib encroaching into the space occupied by the driving surfaces of the corresponding recess-tier of a screw in said range and when the driver is engaged with said recess, whereby engagement of the driver with the recess causes deformation of said driving surface by said rib and hence creation of an interference fit of said driver in said recess.
 16. A system according to claim 1, wherein the smallest tier of said driver comprises a distal end thereof and a proximal end, and said interference comprises a tapering of the cross-section of said smallest tier from said proximal to said distal end, the cross section of the tier intermediate said ends corresponding with the cross section of the smallest recess-tier of a screw in said range, whereby, when the driver is engaged with the recess of a screw, the driving surfaces of the smallest recess-tier of the screw are deformed creating an interference fit between them.
 17. A system according to claim 16, wherein said tiers and recess-tiers are polygonal in section and concentric, said tapering being around the entire periphery of the tier or recess-tier, as the case may be.
 18. (canceled)
 19. A system according to claim 1, wherein the smallest recess-tier of the recess of each screw in said range has a bottom end and an open top end, said interference comprising a tapering of the cross-section of said smallest recess-tier from said open top end to said bottom end, the cross section of the recess-tier intermediate said ends corresponding with the cross section of the smallest tier of said driver.
 20. A system according to claim 19, wherein said tiers and recess-tiers are polygonal in section and concentric, said tapering being around the entire periphery of the tier or recess-tier, as the case may be.
 21. A screw for a system according to claim 1, which screw includes a recess-tier having said rib.
 22. A screw according to claim 21, made by a cold forming process wherein the recess is formed by a punch of corresponding shape to the recess being formed.
 23. A screw according to claim 22, wherein said recess rib is formed by a groove formed in the flank of said punch.
 24. A driver for a system according to claim
 15. 25. A driver for a system according to claim
 16. 26. A screw and driver system comprising a range of screws and a driver for said range, each screw comprising a head having a driving recess in its surface for engagement by said driver and a longitudinal axis, in which the recess of larger screws in the range have a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier having substantially parallel driving surfaces substantially parallel said longitudinal axis, and in which said driver and recess are shaped so that, when the driver is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces; wherein, an interference is provided between the driver and at least one recess-tier causing deformation of the recess when the driver is inserted therein; wherein the smallest tier of said driver comprises a distal end thereof and a proximal end, and said interference comprises a tapering of the cross-section of said smallest tier from said proximal to said distal end, the cross section of the tier intermediate said ends corresponding with the cross section of the smallest recess-tier of a screw in said range, whereby, when the driver is engaged with the recess of a screw, the driving surfaces of the smallest recess-tier of the screw are deformed creating an interference fit between them.
 27. A driver for the system of claim 26, the driver having a longitudinal axis and an end, and a plurality of superimposed tiers of increasing size with increasing distance from said end, each tier having substantially parallel driving surfaces substantially parallel said longitudinal axis, and whereby, when the driver is engaged with the recess of a screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces, wherein the smallest tier of said driver comprises a distal end thereof and a proximal end, the cross-section of said smallest tier tapering from said proximal to said distal end.
 28. A driver according to claim 27, in which said tapering is straight.
 29. A system according to claim 26, wherein said tiers are a non-circular polygon in section.
 30. A system according to claim 29, wherein said polygons are concentric.
 31. A system according to claim 29, wherein said tiers are the same polygons.
 32. A system according to claim 31, wherein said tiers are angularly offset with respect to one another.
 33. A screw and driver system comprising a range of screws and a driver for said range, each screw comprising a head having a driving recess in its surface for engagement by said driver and a longitudinal axis, wherein the recess of larger screws in the range have a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier being polygonal in section having substantially parallel driving flanks substantially parallel said longitudinal axis, and wherein said driver and recess are shaped so that, when the driver is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces; wherein, at least one recess-tier of at least smaller screws in said range has ribs parallel said longitudinal axis disposed in the corners between the flanks of said one recess-tier and encroaching into the space of said recess-tier occupied by said driver when it is engaged with said recess, whereby engagement of the driver with the recess causes deformation of said ribs and hence creation of an interference fit of said driver in said recess. 